Rotating drums have been used for the storage of data in electronic digital computing systems since the first electronic digital computer built in the 1930's. Although magnetic recording superseded these early drums, the use of rotatable drum memories has persisted. The general configuration of a rotating drum memory is a cylinder which is rotated at a constant speed around its axis. A recording medium, such as a magnetic recording material, is deposited on the drum surface. Data is recorded, usually in bit form, by a recording device located adjacent the rotating drum surface. The data is fed from a data source such as a digital computer, and is recorded in circumferential lines, called data tracks, on the drum surface. To read the data, a reading device is placed over the data track to feed the data back to the computing system as the drum rotates. The time required to find and read a particular item of data on the rotating drum is the access time.
Recording drums are superior to disks in that the surface velocity of a drum is constant over the entire surface whereas with disks the velocity varies with the radius. With disks, the maximum data transfer rate is a function of the innermost ring, and the disk operates at a very low efficiency when at the outermost rings unless the disk speed varies with the changing radius. For example, when the outermost rings are twice as long as the innermost rings, the disk operates at 50% efficiency at the outer rings. Drums operate at a constant velocity and have a higher data transfer rate as they operate at 100% efficiency. Additionally, because the mechanical precision of drums is typically greater than disks, less stringent performance is required of the optical focusing and tracking servos used with optical drums. While the coating of magnetic media and photographic emulsions onto drums is technically and economically feasible, the adaptation of the drum configuration to new recording technology such as optical recording is less feasible.
While the first rotating drum memories used capacitors as the recording medium, and most commercial drum memories use magnetic media, optical memory rotating drums have been disclosed in U.S. Pat. Nos. 3,383,662; 3,408,634; 3,440,119; and 3,500,343. In the optical drum memory disclosed in this group of patents, the outside of a cylinder is coated with a photographic emulsion, and data bits are recorded on the resulting photosensitive surface. Data is read from the cylinder by a microscope and a photodiode. This device provides greater bit density than prior magnetic recording media, and is insensitive to strong magnetic fields. However, this device can only record data photographically; the data must be developed and can not be rewritten. Thus, this device can not interact with associated computers or other digital systems in real time as it is read only.
In optical recording technologies using laser recording, a laser beam is focused to a very small spot to record data onto an optically sensitive coating on a substrate. The substrate is an inert substance on which the optically sensitive layer is coated. The data is immediately readable after recording, without any intermediate processing such as chemical development of latent images. Such recording systems are called direct read after write (DRAW) systems. Such a recording mode is permanent and therefore is not erasable and reusable.
Magneto-optic recording is an erasable, reusable method of laser recording, and uses a tightly focused laser spot to heat an area of magnetic material above its Curie point while subjecting the area to a magnetic field. The size of the recorded data bit is determined by the size of the heated area, and is smaller than the area covered by the magnetic field. Therefore, bit areas much smaller than those achievable with conventional magnetic recording heads can be obtained. The magnetically recorded bits can be read by a laser beam.
Magneto-optic recording can attain bit densities as great as ten times that of rigid disk magnetic media as the data bits have an area on the order of one square micrometer, and the bit and track pitch are of similarly small dimensions. To achieve these levels of resolution, the surface of the recording medium must be extremely smooth, a condition not easily producible in cylindrical form with existing technologies. Moreover, apparatus for locating the data on such a small scale must be extremely precise.
Focusing a beam of light, usually a laser beam, to a sufficiently small spot size to achieve resolution on the order of one micrometer requires the distance from the focusing lens to the recording surface to be held to tolerances of one micrometer. While reading a data track, the recording medium surface inevitably moves in a direction normal to its axis of rotation, thereby changing the lens-to-surface distance. With rotating drums, this surface wandering is expressed as the total distance the surface wanders during one revolution of the drum and is commonly referred to as runout, or total indicated runout (TIR). To compensate for this surface wandering, servo systems maintain focus by adjusting the lens location as the surface wanders. In optical disk recording, surface wandering can be over 100 micrometers, and servo systems can adjust the lens location to a tolerance of less than 1 micrometer at typical rotational speeds. As surface wandering in rotating drum memories is considerably less, a simpler focusing mechanism can be used, while providing greater focus accuracy and higher speeds.
Typical diameters of commercially available rotating drum memories are in the range of 26.7-83.3 cm (10.5-32.8 inches). Maintaining surface wandering to within a few micrometers for these drums requires greater precision than is achievable by conventional manufacturing processes, absent expensive and time-consuming finishing operations. Surface wandering of cylindrical drums is caused by the bearings, the eccentricity of the drum surface, and various surface waves, out-of-roundness, and other defects. Even assuming a stationary drum center, the large drum size contributes to dimensional variations which would lead to surface wandering. Furthermore, the large mass and any vibration-causing unbalance increase surface wandering.
Finally, manufacture is complicated by the high level of smoothness required on the outside of the drum. While techniques for producing smooth surfaces on flat optical recording disks are known, these techniques are not suitable for applying similar surfaces to drums. The smooth flat recording surfaces on optical disk recording media can be achieved by coating a curable polymeric liquid onto a horizontal substrate to form a free surface and allowing the liquid to harden. The coating can be thinned by spinning the horizontal substrate around a vertical axis, before hardening, thereby flinging off excess material by centrifugal force. This coating process, referred to as spin-coating, provides very high quality flat surfaces, but can not coat the outside of a drum, as cylindrical surfaces cannot be made horizontal, and gravity causes sagging and other non-uniformities.
Nonetheless, spinning a liquid layer around a vertical axis can form symmetric curved optical surfaces in a technique known as spin casting. Various aspects of spin casting contact lenses are described in U.S. Pat. Nos. 4,416,837; 4,534,915; 4,637,791 and 4,659,522. In the first patent, a mold is spun and used to spin cast contact lenses. In the second patent, UV light is used to minimize stresses by curing the cast product more rapidly near the center. In the third patent, vibration is reduced during spin casting to prevent surface waves in the lens during curing. The last patent describes spin casting of an annular lens. These patents illustrate that spin casting can form optical quality concave surfaces, and that a spin cast polymeric mold can, in some cases, be used to produce an optical quality convex surface.
However, the lenses produced by spin casting in these patents bear little relation to the optical quality surfaces required in a rotating drum used in laser optical recording. Contact lenses are not right circular cylinders and do not require precise mechanical tolerances of the type required of memory drum components. Moreover, when placed in the eye, contact lenses are covered by liquid layers which coat surface roughness. Optical recording involves no liquid layer and requires a high level of smoothness to prevent the optically sensed signal from being lost in noise generated by roughness.